The present invention relates to metal halide vapor discharge lamps, and is more particularly directed to lamps that have efficacies in excess of 35 lumens per watt, in some cases over 100 lumens per watt, but which operate at low to medium power, i.e., usually under 30 watts, but in some cases up to 40 watts. The present invention is more specifically concerned with quartz tube geometry which, in combination with the electrode structure and the mercury, metal halide, and noble gas fill, makes the high efficacy possible.
Metal halide discharge lamps typically have a quartz tube that forms a bulb or envelope and defines a sealed arc chamber, a pair of electrodes, e.g., an anode and a cathode, which penetrate into the arc chamber inside the envelope, and a suitable amount of mercury and one or more metal halide salts, such as NaI, or ScI.sub.3, also reposed within the envelope. The vapor pressures of the metal halide salts and the mercury affect both the color temperature and efficacy. These are affected in turn by the quartz envelope geometry, anode and cathode insertion depth, arc gap size, and volume of the arc chamber. Higher operating temperatures of course produce higher mercury and metal halide vapor pressures, but can also reduce the lamp life cycle by hastening quartz devitrification and causing tungsten metal loss from the electrodes. On the other hand, lower operating temperatures, especially near the bulb wall, can cause salt vapor to condense and crystallize on the walls of the envelope, causing objectionable flecks to appear in objects illuminated by the lamp.
Many metal halide discharge lamps of various styles and power ranges, and constructed for various applications, have been proposed, and are well known to those in the lamp arts. Lamps of this type are described, e.g. in U.S. Pat. Nos. 4,161,672; 4,808,876; 3,324,332; 2,272,647; 2,545,884 and 3,379,868. These are generally intended for high-power applications, i.e., large area illumination devices or projection lamps. It has not been possible to provide a small lamp of high efficacy that could be used in a medical examination lamp or other application at a power of under 40 watts. No one has previously approached lamp building with a view towards applying heat management principles to produce a lamp that would operate a low power and high efficacy and would also develop sufficient mercury and metal halide vapor pressures within the arc chamber without causing devitrification and softening of the quartz tube envelope, and without causing damage to the tungsten electrodes.